A digitizing tablet is a part of a digitizing device that, in conjunction with a stylus, is used to generate digital signals representative of the position of the stylus on the tablet. Digitizing devices are often used with data processing equipment to allow the operator to input information that can best be expressed in multi-dimensional analog terms. For instance, a digitizing device can be used in conjunction with computer aided design equipment to enable the operator to input where features of the item being designed should be located. Digitizing devices can also be used as part of a communications system to enable the users to transmit and receive digital representations of analog information; for example, a digitizing device can be used to transmit handwritten material such as a signature for verification by comparison to a standard stored in a database.
A typical digitizing tablet is provided with two sets of parallel grid conductors. The individual grid conductors are spaced apart from each other and the sets are arranged perpendicular to each other so as to form an X-Y grid of rectangular sections separated by the conductors. The digitizing device includes a digitizing circuit associated with the conductors and stylus that generates a digital output representative of the position of the stylus on the digitizing tablet. In one type of digitizing circuit, a coil inside the stylus is periodically energized and sensors associated with the grid conductors provide an indication of where the stylus is on the digitizing tablet. In another type of digitizing circuit, the grid conductors are driven in succession and the resultant signal induced in a coil in the stylus is then sensed; the time of occurrence and strength of the sensed signal is then used to provide an indication of the location of the stylus. The signals sensed relative to the X-grid conductors provide a indication of where the stylus is in relation to a Y-reference line; the signals sensed relative to the Y-grid conductors provide an indication of where the stylus is relative to an X-reference line. The combination of the signals sensed relative to both the X and Y grid conductors provides an indication of the location of the stylus on the digitizing pad.
Advances in electronics technology have made it possible to interpolate the position of the stylus between parallel grid conductors. This has made it possible to provide digitizing devices that generate an output of the position stylus on the digitizing tablet with a very high degree of accuracy.
An important consideration in the design of a digitizing device is providing a digitizing tablet with grid conductors that are as parallel as possible with respect to the other conductors in the set, and that have substantially identical electrical characteristics. These considerations are important because the signals sensed and processed by the digitizing circuit relative to identical positions between grid conductors should be identical everywhere on the digitizing tablet in order to consistently generate accurate signals representing the stylus' location.
One of the few types of digitizing tablets that, to date, is provided with grid conductors of substantially identical electrical characteristics and has good parallel geometry, is a wire grid digitizing tablet. A wire grid digitizing tablet includes two sets of parallel wires that are spaced apart and perpendicular to each other. Typically, the wires are encased in a resin layer that hold the wires in place and serves as an electrical insulator between them. The resin layer, with the wires embedded therein, is bonded to a glass substrate. The digitizing tablet is incorporated into a standard digitizer device known in the art. A wire grid digitizing tablet is assembled by first stringing the wires to a frame below which the glass substrate is secured; applying a resin that is in a liquid state to the glass so that the wires are coated; and curing and applying pressure to the resin so a thin, solid layer of wire embedded resin, bonded to the glass substrate, is formed. A more detailed description of wire grid digitizing tablets, and the method for constructing them, is set forth in U.S. Pat. No. 4,513,043 which is incorporated herein by reference.
There are a number of disadvantages associated with wire digitizing tablets. One disadvantage is associated with the curing of the resin after it is applied to the glass substrate. In order to cure the resin, it is exposed to ultra violet light. As a consequence of the resin curing, and its exposure to the ultra violet light, heat, known as exotherm, is released by the resin. The heat causes the resin to shrink which in turn may cause the wires embedded therein to bend or become displaced. Controlling the resin's exposure to the ultra violet light only partially reduces the exotherm and subsequent distortion of the wires embedded within it. Lumps or viscosity inconsistencies within the uncured resin can cause the resin to shrink in an irregular pattern so as to further disrupt the pattern of the wires therein. The fabrication of these digitizer tablets is further complicated because there are slight variations in the chemical composition of the different batches of resin used to form the tablets. Accordingly, the resins from the various batches have differing exotherm rates which contribute to the difficulty of controlling the shrinkage when the individual resin layers tablets are cured.
Moreover, the current digitizing tablets are assembled out of costly components that are very difficult to assemble and that must be assembled together with a significant degree of accuracy. As a result, the current digitizing tablets are relatively expensive to manufacture.
Furthermore, all digitizing tablets must be provided with a means to interconnect the grid conductors to the digitizing circuit. Current digitizing tablets do not include an integral structure or member where such interconnects can readily be made. Consequently, current digitizing tablets are often provided with a relatively expensive interconnect coupler, with a mechanical fastener means attached thereto, to make the necessary connections to the digitizing circuits to which they are connected. Frequently, these interconnect couplers are fragile devices that are prone to break off of the tablets.